1. Field of the Invention
The present invention generally relates to a light measuring device for use in a photographic camera and, more particularly, to the light measuring device particularly for use in a single reflex camera having a lens interchangeable capability.
2. Description of the Prior Art
In a photographic camera having a lens interchangeable capability wherein an exposure value is determined by measuring the incident light flux passing through an interchangeable lens assembly at the full open aperture setting, improvement in the structural details of the light measuring optical system used in a particular photographic camera may bring about an error in the measured light value. If correcting values of the measured light values relative to all the available types of photographic cameras are to be stored in each of the interchangeable lens assemblies, each interchangeable lens assembly must have an increased storage capacity In addition, it is not practical to increase the storage capacity in each interchangeable lens assembly which has been distributed.
In view of the foregoing, the Japanese Laid-open Patent Publication No. 62-86337, published Apr. 20, 1987, discloses a system by which the exposure value determined by measuring the incident light flux passing through a phototaking taking lens assembly can be corrected by a value corresponding to a reciprocal of the distance between the exit-pupil and the image plane.
On the other hand, the Japanese Laid-open Patent Publication No. 56-74226, published June 19, 1981, discloses a system wherein, in a photographic camera equipped with a multi-area light measuring device operable to measure the light reflected from a plurality of regions of the field of view and to calculate a proper measured light output based on the results of measurement at those regions, the results of the measurement at those regions can be corrected using lens information such as, for example, the fully opened aperture value, the number of steps stopped down from fully opened aperture, the focal length and the distance between the exit-pupil and the image plate.
However, the prior art systems discussed above have not been devised with a view to the correction of the measured light value to be effected in the event that the structural details of the light measuring optical system in the camera body are modified for the purpose of improvement. In fact, the prior art systems are merely such that the error occurring in the light measurement with the lens assembly having a different distance from exit-pupil to the image plate is corrected or that the corrected value are calculated with respect to the various regions of the multi-area light measuring device. Because of these reasons, the prior art systems cannot cope with change in structural details of the light measuring optical system and, therefore, no proper corrected value can be calculated.